The present invention relates to a solid state laser which oscillates in a range of an ultraviolet ray.
In a short wavelength region extending from blue in a visible light radiation to ultraviolet rays (400 nm or less), a laser beam which is compact, highly efficient and highly stable has been demanded for use in a high density optical recording device and the like.
Conventionally, in the short wavelength region of 400 nm or less, there are various light sources which serve simply as a light source, but lasers which can oscillate in such a short wavelength region are limited. One such laser is a gas laser such as a nitrogen laser, and an excimer laser. However, gas lasers are large in size and poor in efficiency, and gas laser are only able to oscillate in the form of a pulse having a short duration, so that the gas laser cannot be used in the optical recording. There is also known another laser system in which a laser beam having a long wavelength of 500 nm or more is converted into a harmonic component having a wavelength which is 1/2 or 1/3 of the long wavelength. However, this laser system lowers the efficiency of the laser by two or three orders and thus a large laser is required consequently, this laser system cannot be applied practically.
To spread widely the short wavelength laser, there is necessitated a laser which is compact, can stably oscillate, and is highly efficient. For this purpose, it is desirable that the laser includes a laser oscillating medium of a solid state crystal.
It has been already shown that diamond emits light in a region ranging from the visible radiation to the ultraviolet rays through various excitation methods. (See, example, "Exciton Luminescence of Diamond" H. Kawarada, A. Hiraki, NEW DIAMOND, Vol. 6 No. 3 (1990) p2, "Cathodoluminescence and electroluminescence of undoped and boron-doped diamond formed by plasma chemical vapor deposition" H. Kawarada, Y. Yokota, Y. Mori, k. Nishimura, A. Hiraki, J. Appl. Phys., 67 (1990) p983, "Blue and green cathodoluminescence of synthesized diamond films formed by plasma-assisted chemical vapor deposition," H. Kawarada, K. Nishimura, T. Ito, J. Suzuki, K. Mar, Y. Yokota, Jap. J. Appl. Phys. 27 (4) (1988) pL683.
Also, there is proposed a method in which a color center is formed by introducing impurities or defects into a diamond crystal in order to produce a solid state laser in a visible radiation region. (See example, Japanese Patent Unexamined Publication No. Sho.63-246885, Japanese Patent Unexamined Publication No. Sho.64-20689).
As described above, the diamond is very stable as a material, provides a high degree of transparency and has a great thermal conductivity, so that the diamond is excellent as a medium for the oscillation of an ultraviolet solid state laser. For this reason, the practical application of the diamond has been demanded. However, the diamond laser oscillation in the ultraviolet range has never been attained practically.